Surface organometallic chemistry on metals. Part IV. Selective hydrogenation of ethyl acetate to ethanol on RhSn/SiO₂ bimetallic catalysts: A mechanistic study

Bimetallic catalysts RhSn, NiSn and RuSn, prepared via an organometallic route, are very selective for the hydrogenolysis of ethyl acetate to ethanol. This generation of catalysts, requires lower temperatures and lower pressures than the traditional copper chromites and constitutes an elegant method for the hydrogenolysis of (fatty) esters into (fatty) alcohols, which are important raw materials derived from natural resources. The objective of this work was to identify, via kinetic experiments, the primary products of the reaction in order to propose a mechanistic explanation for the selective hydrogenation of ethyl acetate to ethanol. Changing the Sn/Rh ratio drastically modifies both the activity and the selectivity of the catalysts: In the range 0 < Sn/Rh < 0.2, both the activity and the selectivity for hydrogenolysis to CO and CH₄ decrease. On the other hand, for Sn/Rh ratios higher than ca. 0.2, both the activity and the selectivity for the ethanol formation increase almost linearly with the tin content while the rates of alkane and CO formation remain constant and very low. At Sn/Rh = 1.7, kinetic studies indicate that ethanol, acetaldehyde and ethane are the primary products, whereas CO and CH₄ are secondary products. The rate low, which is of the form: r = k(P_H2)^0.5K_EAP_EA/(1 + K_EAP_EA + K_EP_E). is totally different from that observed on pure rhodium. These kinetic data, associated with previous physical observations leading to the concept of 'site isolation' of rhodium atoms in a matrix of tin atoms, lead to the proposal of a reaction mechanism involving a single rhodium atom.